A typical electric power cable generally comprises a conductor that is surrounded by several layers of polymeric materials including an inner semiconducting layer, followed by an insulating layer, and then an outer semiconducting layer. These layers can be crosslinked as well known in the field. To these layers, one or more further auxiliary layer(s) may be added, such as a screen and/or a jacketing layer(s) as the outer layer. The layers of the cable are based on different types of polymers. E.g. low density polyethylene, crosslinked by adding peroxide compounds, is a typical cable insulation material.
Polyolefins, particularly polyethylene is generally used as an electrical insulation material as it has good dielectric properties, especially high breakdown strength. Such insulated cables have, however, a drawback that they suffer from shortened service life when installed and operated in an environment where the polymer is exposed to water, e.g. under ground or at locations of high humidity. The reason is that polyolefins tend to form bush-shaped defects, so-called water trees, when exposed to water under the action of electrical fields. Such defects may be due to inhomogeneities, for instance microcavities and impurities, such as contaminants, occurring in the layer material and they can lead to lower breakdown strength and possibly electric failure when in use.
The appearance of water tree structures are manifold. In principle, it is possible to differentiate between two types:                “Vented trees” which have their starting point on the surface of the semiconductive layer(s) and        “Bow-tie trees” which are initiated within the insulation material often starting from a defect or a contaminant.        
Water treeing is a phenomenon that has been studied carefully since the 1970's. Many solutions have been proposed for increasing the resistance of insulating materials to degradation by water-treeing. One solution involves the addition of polyethylene glycol, as water-tree growth inhibitor to a low density polyethylene such as described in U.S. Pat. No. 4,305,849 and U.S. Pat. No. 4,812,505. Furthermore, the invention WO 99/31675 discloses a combination of specific glycerol fatty acid esters and polyethylene glycols as additives to polyethylene for improving water-tree resistance. Another solution is presented in WO 85/05216 which describes copolymer blends.
Further, EP 1 695 992 describes at least one ether and/or ester group containing additive that is combined with an unsaturated polyolefin in order to provide a polyolefin composition with enhanced crosslinking properties and improved water-tree resistance.
Accordingly, there is a continuous need to find alternative solutions with advantageous water tree resistance properties to overcome the prior art problems.